Iron, ruthenium, and osmium complexes supported by the bis(silyl) chelate ligand (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl): Synthesis, characterization, and reactivity

The bis(silyl)-type bidentate ligand precursors xantsil-H₂ (la) and 2,7-di-t-butylxantsil-H₂ (1b) possessing the xanthene backbone were prepared by dilithiation of the 4,5-positions of 9,9-dimethylxanthene or 2,7-di-t-butyl-9,9-dimethylxanthene using n-BuLi in the presence of tetramethylethylenediamine (TMEDA) followed by treatment with chlorodimethylsilane. According to X-ray diffraction analysis of lb, the xanthene core is close to planar as observed in the dihedral angle of 6.2(2)° between two least-square planes of two aromatic rings in xanthene. UV irradiation of [Fe(CO)₅] and la in dichloromethane provided m-[Fe(xantsil)(CO)₄] (2), while thermal reactions of [M ₃(CO)₁₂] (M = Ru and Os) and la provided cis-[M(xantsil)(CO)₄] (M = Ru (3) and Os (4)). In the course of the synthetic study on 3, formation of [Ru₃(xantsil)(μ-H) ₂(CO)₁₀] (5) was confirmed and independently synthesized by the reaction of [Ru₃(CO)₁₀(CH₃CN) ₂] with la. Thermolysis of 5 and la at 120 °C for 13 min afforded 3, indicating its intermediacy to 3. Refluxing the toluene solution of 3 for 3 h resulted in the replacement of three carbonyl ligands with toluene to give [Ru(xantsil)(CO)(η⁶-toluene)] (6). Dissociation of the three carbonyl ligands would be enhanced by the severe steric repulsion between the SiMe₂ moiety and the three fac-carbonyl ligands, high trans effect of silyl groups, and precoordination of the xanthene oxygen atom.